1. Field of the Invention
The invention relates to the field of integrated circuit testing.
2. Prior Art
The testing of integrated circuits at the time they are manufactured and before they are incorporated into a larger assembly is very necessary. A microprocessor, for instance, should be carefully tested before it is incorporated into a computer not only to avoid a costly computer repair but also since it becomes more difficult to test the microprocessor after it is installed in a computer.
Several integrated circuit testing techniques are summarized in Designer's Guide to Testable ASIC Devices, by Wayne Needham, published by Van Nostrand Reinhold, Chapters 4 and 5. Also see U.S. Pat. Nos. 3,643,156; 3,927,371 and 4,635,218.
The vast majority of microprocessors as well as other integrated circuits are tested with a stored response tester. These testers contain a large memory that stores patterns of ones and zeros used as inputs to the microprocessor along with patterns of the correct outputs expected from the microprocessor. Patterns may be applied to a device under test (DUT) through the normally used input terminals or through special scan-in, scan-out ports. A subset of the test vectors used to verify the microprocessors design is often selected for this testing. By way of example, where a hundred million vectors are used to verify the integrated circuit's design, 10 million may be used for the testing of each microprocessor during manufacturing.
A microprocessor with three-or so million transistors is generally tested for 15-25 seconds after being manufactured by a stored response tester costing several million dollars. (Part of this testing is done by probing a wafer and part after the microprocessor has been packaged.) Consequently, many testers are required for a high volume integrated circuit and the cost of the testers is a significant part of the total capital cost required to fabricate some very large scale integrated circuits.
As the number of transistors in an integrated circuit increases, the required testing time also increases. For instance, for a microprocessor with six to seven million transistors, an estimated 30-45 seconds will be required again on a stored response tester costing several million dollars. Indeed, it is estimated that using current testing methodologies, the cost of testing a microprocessor with 10 million transistors, will equal or exceed the cost of producing the die (without packaging).
As will be seen, the present invention substantially reduces testing costs.